This invention relates to an AM radio receiver having a scan-tuning feature and more particularly to such a receiver including an FM discriminator followed by a "window" level detector for producing a scan stop signal only when a station is in tune.
A scan-tuning feature is known whereby, upon being initiated manually, the radio is automatically scan-tuned from one end of the broadcast band toward the other and stopping on a station of signal strength exceeding a predetermined satisfactory value. Since AM broadcast stations are allocated, their carrier frequencies, each in the middle of a narrow 10 KHz wide sub-band or channel, the scan-tuner of an AM receiver may step-tune the receiver in 10 KHz increments through the broadcast band.
Scan-tuning circuits are employed in automobile radios to ameliorate the problem of constant changes in station signal strength that requires frequent searching for a station giving high quality reception. The scanner typically monitors the (automatic gain control) AGC voltage, which is a measure of radio signal strength, and stops in the middle of the 10 KHz band for which there is being received a signal of acceptable strength. In other words, it should stop only when the AGC signal is greater than a predetermined value corresponding to an acceptably strong radio signal. Such a scanner is described in the paper entitled One Chip AM/FM Digital Tuning System by Ichinose et al, IEEE Transactions on Consumer Electronics, Vol. CE - 26 Aug. 1980.
However, as the scanner approaches a very strong station some signal is detectable well before reaching the station carrier frequency. Thus, such automatic scan-tuning systems tend to stop a whole 10 KHz increment early before reaching the frequency of a very strong station. The initial result is highly distorted disagreeable sound.
It is also known to combine with such automatic scan-tuning radios, an automatic frequency control (AFC) circuit that is actuated after the scanning is stopped. Such AFC systems, however, cannot span the 10 KHz range necessary to change the tuning to the next allocated station frequency.
It is therefore an object of this invention to overcome the above-noted short comings of the prior art.
It is a further object of this invention to provide an AM radio receiver having an automatic scan-tuning system that does not stop on a signal unless that signal is of adequate strength and is also accurately tuned in.